One trap that developers need to be wary of is the mentality of the Hammer Truism. This states that
When the only tool you have is a hammer, everything looks like a nail.
This is especially true of Design Patterns. I particularly liked what Erich Gamma said in this interview
Do not start immediately throwing patterns into a design, but use them as you go and understand more of the problem. Because of this I really like to use patterns after the fact, refactoring to patterns.
All too often, I’ve encountered code that uses a pattern because the developer felt he should use a pattern there, not because he needed the pattern.
Not every developer understands that patterns add complexity to a solution. Certainly abstraction and redirection are important benefits of many design patterns, but they come at a cost. To use design patterns effectively is to know when the benefits will payback that cost with interest.
The important concept to understand is that Design Patterns are descriptive not prescriptive. They aren’t intended to instruct how one should design a system, but merely describe successful designs that have worked in the past for common problems. Should the problem you’re tackling fit a particular recurring pattern, then applying a design pattern is certainly a good choice. But when the problem doesn’t quite fit one of the patterns, trying to cram the round pattern into the square design just doesn’t fit.
I’ve recently seen an example of this in regards to using an interface. I generally follow the rule of threes regarding polymorphism. For example, if I have a class with an enum indicating its "type" (for example, a User class with an enum property indicating whether the User is an employee or a manager), when that enum contains three values, I’ll consider refactoring the class to have a base class and inherited classes (for example, the User class might have an Employee subclass and Manager Subclass). Maybe I'll use an IUser interface instead.
However, I caution against using an interface (or inheritance) just because it’s the "right" thing to do. There’s no point to implementing polymorphism if it is never used.
For example, I recently saw several classes in some code I was reading that implemented an interface we’ll call ISomeInterface. But nowhere did I find any code that referenced ISomeInterface. Instead, there were only references to concrete classes. I expected to see something like this somewhere in the code.
foreach(ISomeInterface something in SomeInterfaceCollection)
But no such code could be found. This was a prime example of a gratuitous use of an interface. This interface served no purpose and needed to be removed.
The important lesson here is to always start off by writing the simplest code possible and only add interfaces and design patterns when they are absolutely needed.